CRISPRs (Clustered Regularly Interspaced Short Palindromic Repeats) evolved in bacteria as an adaptive immune system to defend against viral attack. Upon exposure to a virus, short segments of viral DNA are integrated into the CRISPR locus. RNA is transcribed from a portion of the CRISPR locus that includes the viral sequence. That RNA, which contains sequence complimentary to the viral genome, mediates targeting of a Cas9 protein to to a target sequence in the viral genome. The Cas9 protein cleaves and thereby silences the viral target.
Recently, the CRISPR/Cas system has been adapted for genome editing in eukaryotic cells. The introduction of site-specific double strand breaks (DSBs) allows for target sequence alteration through one of two endogenous DNA repair mechanisms—either non-homologous end-joining (NHEJ) or homology-directed repair (HDR). The CRISPR/Cas system has also been used for gene regulation including transcription repression and activation without altering the target sequence. Targeted gene regulation based on the CRISPR/Cas system uses an enzymatically inactive Cas9 (also known as a catalytically dead Cas9).
Despite the recent advances adapting the CRISPR/Cas system for genome editing in eukaryotic cells, there remains a need for improved regulation and control of these systems for use in eukaryotic cells.